1. Field of the Invention
This invention is related to an optical projection reduction system for use with short wavelength radiation in photolithography equipment used in the manufacture of semiconductor devices.
2. Background of the Invention
Photolithography is a well known manufacturing process used to create devices upon substrates. The process typically involves exposing a patterned mask to collimated radiation thereby producing patterned radiation, which is passed through an optical reduction system. The reduced patterned radiation or mask image is projected onto a substrate coated with photoresist. Radiation exposure changes the properties of the photoresist, allowing subsequent processing.
Exposure tools used in photolithography have two common methods of projecting a patterned mask onto a substrate: "step and repeat" and "step and scan." The step and repeat method sequentially exposes portions of a substrate to a mask image. The step and repeat optical system has a projection field that is large enough to project the entire mask image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
In contrast, the step and scan method scans the mask or reticle onto a wafer substrate over an annular field or a slit field that is the full height of one or more of the chips. Referring to FIG. 1, a ring field lithography system 100 for use in the step and scan method is shown. A moving mask 101 is illuminated by a radiation beam 103, which reflects off the mask 101 and is directed through a reduction ring field optical system 107. Within the optical system 107, the image is inverted and the arcuate shaped ring field 109 is projected onto a moving substrate 111. The arcuate shaped reduced image ring field 109 can only project a portion of the mask 101, thus the reduced image ring field 109 must scan the complete mask 101 onto the substrate 111. Because the mask 101 and substrate 111 move synchronously, a sharp image is scanned onto the substrate 111. Once the complete mask 101 is scanned onto the substrate, the mask 101 and substrate 111 are repositioned and the process is repeated. The dimensions of the slit are typically described by a ring field radius and a ring field width.
As manufacturing methods improve, the minimum resolution dimension or critical dimension (CD) which can be achieved decreases, thereby allowing more electronic components to be fabricated within a given area of a substrate. The number of devices that can be fabricated within an area of substrate is known as device density. For example, a common measure of device density is the amount of memory that can be fabricated on a single DRAM chip. As resolution dimension or CD decreases, DRAM memory size increases dramatically. With existing technology, 0.25 .mu.m resolution is possible.
One well-known means of improving the resolution dimension and increasing device density is to use shorter exposure wavelength radiation during photolithography processes. The relationship between resolution dimension and radiation wavelength is described in the formula: R=(K.sub.1 .lambda.)/(NA), wherein R is the resolution dimension, K.sub.1 is a process dependent constant (typically 0.7), .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the optical system projecting the mask image. Either reducing the wavelength or increasing the NA will improve the resolution of the system.
Improving the resolution by increasing the numerical aperture (NA) has several drawbacks. The most prevalent drawback is the concomitant loss of depth of focus with increased NA. The relationship between NA and depth of focus is described in the formula: DOF=(K.sub.2 .lambda.)/NA.sup.2, wherein DOF is depth of focus, and K.sub.2 is a process dependent constant (typically close to unity). This simple relationship shows the inverse relationship between DOF and NA. For several reasons, including practical wafer flatness and scanning stage errors, a large depth of focus is on the order of .+-.1.0 micrometers is desirable.
Immediately, the shortcomings of resolution improvement via numerical aperture increase can be seen. As lithography technologies evolve toward shorter wavelengths, projection systems operate in different regions of wavelength-NA space. For EUV lithography at an operational wavelength of 13.4 nm, 0.1 .mu.m resolution can be achieved with a projection system that has a numerical aperture of 0.10 (assuming K.sub.1 =0.7). A depth of focus of at least .+-.1.0 .mu.m results from this low numerical aperture. This large depth of focus will enhance the robustness of a particular lithographic process. In contrast, deep ultraviolet (DUV) lithography at a wavelength, l , of 193 nm requires a projection system with a numerical aperture of 0.75 to achieve 0.18 .mu.m features (assuming K.sub.1 =0.7). At this NA, the depth of focus has been reduced to .+-.0.34 .mu.m. This reduction in depth of focus leads to a loss in available process window, which will adversely impact process yield. As the process shrinks, it becomes more difficult to maintain the CD control that is critical to the lithographic process.
As is known in the art, short l radiation (less than about 193 nm) is not compatible with many refractive lens materials due to the intrinsic bulk absorption. To reduce the radiation absorption within an optical system, reflective elements may be used in place of refractive optical elements. State of the art DUV systems use catadioptric optical systems that utilize a combination of refractive and reflective optical elements. Since the mirrors provide the bulk of the optical power, the use of refractive lens elements is minimized
To produce devices with smaller critical dimensions and higher device density than is possible with DUV systems, optical systems compatible with even shorter wavelength radiation are required. Extreme ultraviolet (EUV) radiation (l less than about 20 nm) offers the potential to reduce the critical dimension from the current state of the art of 0.18 mm to below 0.05 mm. This radiation cannot be focused refractively. However, EUV radiation can be focused reflectively using optical elements with near normal incidence multilayer coatings.
Early development of optical systems for EUV projection lithography concentrated on projection of two-dimensional (2D) image formats at low numerical apertures. One example of a step and repeat optical system is disclosed in U.S. Pat. No. 5,063,586. The '586 patent discloses coaxial and tilted/decentered configurations with aspheric mirrors that project approximately a 10 mm.times.10 mm image field. The '586 patent system achieves an resolution of approximately 0.25 .mu.m across this field, but suffers from unacceptably high distortion, on the order of 0.8 .mu.m. The optical system described by the '586 patent is impractical because the mask would have to pre-distorted in order to compensate for the distortion in the projection optics.
More advanced optical systems for EUV projection lithography evolved using the step and scan image partitioning method in response to the unacceptable distortion found in the large format step and repeat optical systems. Step and scan systems have inherently less distortion than step and repeat systems due to the reduced field size in the scan dimension. The distortion can be readily corrected to acceptable levels over the field in the scan dimension. Step and scan optical systems typically utilize ring fields. Referring to FIG. 2, in a step and scan optical system an image is projected by the optical system onto the wafer through an arcuate ring field slit 201 which is geometrically described by a ring field radius 203, a ring field width 205 and a length 207. Ring field coverage is limited to 180.sub.i in azimuth.
One example of a step and scan optical system is disclosed in U.S. Pat. No. 5,315,629. Although the '629 patent optical system has low distortion, it does so at the expense of ring field width. The ring field slit width is only 0.5 mm at the wafer. High chief ray angles at mirror M1 make it difficult to widen the ring field width at a usable numerical aperture. The 0.5 mm ring field width of the '629 patent limits the speed at which the wafer can be scanned, restricting throughput. The ring field radius of the optical system described in the '629 patent is 31.5 mm, which limits the field width in the cross scan dimension. Like the '586 patent, the '629 patent is best suited for critical dimensions on the order of 0.1 mm. The numerical aperture of the optical system in the '629 can be scaled up to achieve higher resolution. However, the ability to control distortion is lost as the numerical aperture is scaled to larger values.
Another example of a step and scan optical system is U.S. Pat. No. 5,353,322. The '322 patent discloses 3-mirror and 4-mirror optical systems for EUV projection lithography. An optical system with an odd number of reflections necessitates that the mask and wafer be located on the same side of the optical system. Thus, the motion of the stages that carry the mask and wafer are limited. An extra fold mirror added to the 3-mirror embodiment found in the '322 patent creates a 4-mirror system that enables unlimited stage travel since the wafer and mask are now on opposite sides of the optical system. However, this extra fold mirror does not provide any optical power and thus provides no aberration correction. The principle drawback of the '322 optical system is that its aperture stop is physically inaccessible. Although the stop location allows for a numerical aperture of up to 0.125, the projected imagery could vary substantially across the ring field as the various hard apertures vignette light diffracted by the mask features in the optical system. This is due to the fact that these systems have no physically accessible hard aperture stop to define the imaging bundles from each field point in a like manner. If this vignetting is field dependent, it can lead to loss of CD control across the projected ring field.
Clearly, state of the art optical systems for EUV projection lithography can be used to resolve features sizes that are on the order of 0.1 mm (100 nm). As demonstrated, these systems are coaxial 3- and 4-mirror reflective anastigmats that are optimized to operate over a narrow ring field. Since it is difficult to control both the field and pupil dependence of the aberrations simultaneously, the numerical aperture of these systems is necessarily restricted to approximately 0.10 for ring field of any substantial width (0.5 mm to 1.5 mm).
Prior art offers no concrete examples of multi-mirror systems that achieve higher NAs (&gt;0.15) with low static distortion (&lt;CD/10). Examples of optical systems for EUV projection lithography with numerical apertures in excess of 0.10 are disclosed in U.S. Pat. No. 5,212,588. The '588 patent demonstrates a multi-bounce projection system that incorporates 2 coaxial aspheric mirrors in a 4-bounce configuration. Mirror M1 is convex and mirror M2 is concave. Both mirrors have substantially the same radius of curvature to obtain a near zero Petzval sum. This ensures that high resolution imagery will be obtained on a flat imaging surface. While the '588 patent describes a number of embodiments with excellent performance over a range of numerical aperture up to 0.17 at EUV wavelengths, all the systems suffer one common flaw: the exit pupil is centrally obscured by mirror M1. This central obscuration suppresses the MTF response of the system at the mid-spatial frequencies relative to the cut-off frequency. Since the obscuration is large (on the order of 40%), this loss of contrast will yield unacceptable lithographic imaging performance.
One path to higher resolution is to add an extra mirrored surface in such a manner as to enhance the simultaneous correction of both the field and pupil dependence of the aberrations. An examples of a 5-mirror EUV projection system is disclosed in U.S. Pat. No. 5,153,898. However, only three of the mirrored surfaces provide aberration correction, and the extra two mirrors only act to fold or redirect the incoming radiation. The '898 patent does not enable high numerical optical systems for EUV projection lithography.
In view of the foregoing, there is a need for an optimized optical system which is compatible with short wavelength radiation and has a high numerical aperture for improved resolution.